Yes, he is technically correct along that line. However, it depends on what you define "flat" as. The current generally accepted definition is something that only has 2 dimensions or a 3-D model that represents 2-D. For example, a piece of paper is a 3-D object but it figuratively represents a 2 dimensional object.
Your friend physicist was just playing around the definition of "flat".
The Laminator
For goodness's sake, please vote Democrat this November!